1. Field of the Invention
The present invention relates to sheet conveying control and loading processing in a sheet processing apparatus to be connected to an image forming apparatus.
2. Description of the Related Art
Hitherto, some image forming apparatuses, such as printers, have been equipped with a sheet processing apparatus that aligns end portions of a sheet bundle comprising a plurality of sheets with images formed thereon (or printed sheets) and that performs post-processing such as stapling (i.e., needle driving) before discharging the sheet bundle.
Such a sheet processing apparatus has had an arrangement wherein sheets are conveyed onto a stack tray used for performing stapling, wherein, after a predetermined number of sheets have been conveyed to thereby form a sheet bundle, the sheet bundle is stapled, and wherein the stapled sheet bundle is discharged onto a paper discharge tray by driving paper discharge rollers.
In the sheet processing apparatus with such features, since the stapling portion of each of the sheet bundles protrudes, when large numbers of sheet bundles are discharged onto a paper discharge tray, stapling portions formed for each sheet bundles are superimposed one on another, thereby forming a protuberance at a local part of the sheets loaded on the paper discharge tray. This protuberance unfavorably causes the sheet processing apparatus to erroneously detect a full load, i.e., the upper limit of loading of sheet bundles, although more sheet bundles can be loaded onto the paper discharge tray.
Accordingly, Japanese Patent Laid-Open No. 2000-95420 discloses an invention that prevents such a local protuberance in sheets loaded on a paper discharge tray by discharging sheet bundles so that staple portions thereof are not superimposed on each other.
However, in the invention set forth in the Japanese Patent Laid-Open No. 2000-95420, the arrangement is such that stapling is performed with respect to sheet bundles loaded on upper trays, that the upper trays are moved by a predetermined distance along the direction perpendicular to the sheet bundle discharge direction, and that sheet bundle is let to fall from the upper trays onto a lower tray, thereby performing loading in a manner such that the stapling positions of sheet bundles loaded on the lower tray are displaced from each other. Therefore, in this arrangement, it has not been possible to discharge paper in a state where the stapling positions of sheet bundles are displaced from each other along the sheet bundle discharge direction, although it has been possible to discharge paper in a state where the stapling positions of sheet bundles are displaced from each other along the direction perpendicular to the sheet bundle discharge direction.